Abstract
Animals adaptively regulate their metabolic rate and hence energy expenditure over the annual cycle to cope with energetic challenges. We studied energy management in greylag geese. In all geese, profound seasonal changes of heart rate (fH) and body temperature (Tb) showed peaks in summer and troughs during winter, and also daily modulation of fH and Tb. Daily mean fH was on average 22% lower at the winter trough than at the summer peak, whereas daily mean Tb at the winter trough was only about 1 °C below the summer peak. Daily means of Tb together with those of air temperature and day length were the most important predictors of daily mean fH, which was further modulated by precipitation, reproductive state, and, to a minor degree, social rank. Peaks of fH and Tb occurred earlier in incubating females compared to males. Leading goslings increased daily mean fH. Our results suggest that in greylag geese, pronounced changes of fH over the year are caused by photoperiod-induced changes of endogenous heat production. Similar to large non-hibernating mammals, tolerance of lower Tb during winter seems the major factor permitting this. On top of these major seasonal changes, fH and Tb are elevated in incubating females.
Highlights
Tb during winter seems the major factor permitting this
Torpor or hibernation are strategies for down-regulating metabolic rate (MR) and Tb, thereby adjusting energy management when faced with food shortage and/or low ambient temperatures
In birds, reduced basal metabolic rate (BMR)[12], and associated facultative hypothermia occurs in many species as a reaction to food shortage or low ambient temperatures; one species, the common poorwill (Phalaenoptilus nuttallii) is even known to hibernate
Summary
Tb during winter seems the major factor permitting this. On top of these major seasonal changes, fH and Tb are elevated in incubating females. Torpor or hibernation are strategies for down-regulating metabolic rate (MR) and Tb, thereby adjusting energy management when faced with food shortage and/or low ambient temperatures (for review see[3,4]). Birds are expected to have higher energy expenditure due to increased thermoregulatory needs This additional energy expenditure is, according to the ‘reallocation hypothesis’, reallocated to reproductive activity during the breeding season, because birds breed when ambient temperatures are moderate[22]. Based on what is known about the winter physiology of non-hibernating mammals, we here suggest ‘winter hypometabolism’ as a third hypothesis for explaining seasonal changes of MR For large birds, such as geese, this hypothesis predicts a decrease in MR during winter mainly due to reduced endogenous heat production and a tolerance of lower Tb8,24, during the nocturnal rest phase[5,7]. In contrast to previous studies which mostly considered changes in activity patterns and energy content of food as drivers for changes in MR, we here predict that a change of endogenous heat production, accompanied by a down-regulation of Tb, is the most important modulator of MR
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